Intermetallic materials have been of considerable interest and have undergone increasing development over the past several years, especially the intermetallics of aluminum such as the aluminides of titanium, zirconium, iron, cobalt, and nickel. The need for the advanced properties obtainable with intermetallic materials is typified by their potential application to structures capable of withstanding high temperatures, such as turbine engines. In designing and operating turbine engines today, and for the foreseeable future, there are two primary problems which demand solutions from the field of materials science. The first of these is the need to operate certain portions of the engine at higher temperatures to improve operating efficiency and save fuel. The second problem is the need for lighter materials to decrease engine weight and engine operating stresses due to heavy rotating components, and to increase the operating life of disks, shafts, and bearing support structures. These latter structures require materials which are less dense than conventional nickel base superalloys, but which possess roughly the same mechanical properties and oxidation resistance as those materials in current usage.
Intermetallic compounds are particularly suited to these needs because of properties which derive from the fact that they possess ordered structures having regularly repeating (e.g., A B A B A B) atom sequencing. Modulus retention at elevated temperature in these materials is particularly high because of strong A-B bonding. In addition, a number of high temperature properties which depend on diffusive mechanisms, such as creep, are improved because of the generally high activation energy required for self-diffusion in ordered alloys.
The formation of long range order in alloy systems also frequently produces a significant positive effect on mechanical properties, including elastic constants, strength, strain-hardening rates, and resistance to cyclic creep deformation. Finally, in the case of titanium aluminides, the resistance to surface oxidation is particularly good because these materials contain a large reservoir of aluminum that is preferentially oxidized.
However, during metallurgical processing, one problem encountered is that these materials tend to form coarse grains, which adversely effect workability, and which degrade certain mechanical properties, the most important of which is ductility. Also, in many intermetallics the strong A-B bonding results in low temperature brittleness, although the exact mechanism of the ductile-brittle transition seems to be different for the different intermetallic compounds. It is thus necessary to address the problem of minimal low temperature ductility without destroying the inherent high temperature strength and stiffness. In the prior art it has generally been considered that these latter high temperature properties may only be retained by preserving the ordered structure. However, little progress has been made in developing practical intermetallic compositions that possess sufficiently improved low temperature ductility while maintaining high temperature strength.
Another class of materials which has been extensively developed over the past several years comprises metal-second phase composites, such as aluminum reinforced with carbon, boon, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal-ceramic composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. However, the formation of composites comprising intermetallic matrices has not been widely investigated. Moreover, the formation of high quality composites comprising titanium aluminide matrices having titanium silicide intermetallic particles dispersed therein has not heretofore been achieved.
Conventional powder metallurgy techniques for the production of dispersion-strengthened composites involve the mechanical mixing of metal powders of approximately 5 micron diameter or less with powder of the second phase material (preferably 0.01 micron to 0.1 micron). High speed blending techniques or procedures such as ball milling may be used to mix the powders. Standard consolidation techniques are then employed to form the final composite. Typically, however, the second phase component is large, i.e. greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials since their production is energy intensive, time consuming, and costly in capital equipment. Furthermore, the production, mixing and consolidation of very small particles inevitably leads to contamination at the surface of the particles. Contaminants, such as oxides, inhibit interfacial binding between the second phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact can also result in reduced strength, loss of elongation, and facilitated crack propagation. In addition, the matrix may be adversely effected, as in the case of titanium which is embrittled by interstitial oxygen.
Alternatively, it is known that proprietary processes exist for the direct addition of appropriately coated ceramics to molten metals. Further, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect the ceramic particles from the molten metal during admixture or molten metal infiltration, and to improve bonding between the metal and ceramic. Techniques such as these have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (e.9., greater than 1 micron) and whiskers, because of the high pressures involved for infiltration. In the molten metal infiltration technique, the ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-second phase composites.
The above noted powder metallurgical and molten metal techniques have not been applied to the production of composites comprising titanium silicide particles dispersed in titanium aluminide matrices, due to the fact that titanium silicide powders in the micron size range are not readily available. In addition, it has not been considered practical to utilize titanium silicide particles within titanium containing matrices since titanium is known to be highly reactive, especially at elevated temperatures. Thus, one would expect titanium silicide particles to be highly unstable within a titanium containing environment.
In recent years, numerous ceramics have been formed using a process referred to as self-propagating high-temperature synthesis (SHS), which involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. The SHS process involves mixing and compacting powders of the constituent elements, and igniting the green compact with a suitable heat source. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long times at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al. In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from groups IV, V, and VI of the Periodic System with a non-metal such as carbon, boron, silicon, sulfur, or liquid nitrogen, and locally heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by localized ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, in the absence of a metallic phase.
U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal, such as copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the reaction, resulting in an alloy comprising titanium diboride, titanium carbide, and the binder metal. This reference is limited to the formation of TiB.sub.2 and TiC ceramic materials and to the use of Group I-B metals such as copper and silver, as binders. The process is performed with a relatively high volume fraction of ceramic and a relatively low volume fraction of metal (typically 6 volume percent and below, and almost invariably below 20 volume percent). The product is a dense, sintered material wherein the relatively ductile metal phase acts as a binder or consolidation aid which, due to applied pressure, fills voids, etc., thereby increasing density.
U.S. Patent Application Ser. No. 873,890, filed June 13, 1986, of which this application is a Continuation-In-Part, and which is hereby incorporated by reference, discloses several methods for the production of intermetallic-second phase composites. Emphasis is drawn to the production of composites consisting of aluminide matrices having ceramic particles dispersed therein. While the application does disclose the formation of intermetallic second phase particles within aluminide matrices, no specific disclosure is made of the formation of titanium silicide intermetallic particles within a titanium aluminide containing matrix. The methods taught in the 873,890 application may be adapted for use in the present invention, wherein zirconium alloying additions are utilized during processing to stabilize titanium silicide particles within titanium aluminide containing matrices.
U.S. patent application Ser. No. 190,561, filed May 5, 1988, which is hereby incorporated by reference, discloses a method for the production of intermetallic-second phase composites that is related to the methods taught in the 873,890 application discussed above, but which involves the use of additional processing steps. As in the 873,890 application, the formation of composites comprising intermetallic second phase particles within aluminide matrices is taught. However, no specific disclosure is made of the production of titanium aluminide-titanium silicide composites utilizing zirconium alloying additions. The methods taught in the 190,561 application may also be adapted for use in the present invention, wherein zirconium is used as an alloying addition during processing.
Similarly, U.S. Pat. Nos. 4,710,348 and 4,751,048, and U.S. patent application Ser. Nos. 927,014; 927,031; and 190,547, which are hereby incorporated by reference, disclose various methods for the production of metallic-second phase composites. However, none of these patents and patent applications teach the formation of composites comprising titanium aluminide containing matrices having titanium silicide particles dispersed therein.